The present invention relates to a method and an apparatus for measuring moisture in material, and particularly to an improved apparatus which is less sensitive to the positioning or thickness of the material than prior art devices.
Moisture measuring devices of the prior art typically employ some kind of contacting means for making a conductive connection with material in which moisture is to be measured so that the moisture can be determined by electrical conduction. Unfortunately, the contacting means or brushes are subject to breakage and shorting whereby the moisture indications tend to become inaccurate. Further, even if the brushes are in good condition, the degree of electrical contact provided with the material under test is nonuniform.
Moisture detectors have been developed which do not require contact with the material but instead employ capacitive coupling or the like. Many, however, are quite sensitive to the position of the material relative to the sensor conductor, as well as to the thickness of the material, and therefore indications derived on a production line basis can be somewhat undependable. Also, the conveying means upon which the material is transported can short out the measuring system such that a dependable reading is not obtained.
In prior U.S. Pat. No. 4,377,783, a measuring system is set forth in which transmitting and receiving plates are offset along the path of the measured material, and a conductive path in the veneer and the grounded conveyor is employed as part of the circuit. While very efficacious, there is again some dependency upon accurate contact with the material being measured.
In prior application Ser. No. 494,953 filed May 16, 1983 now U.S. Pat. No. 4,563,635 moisture in wood veneer strips is measured by passing the veneer strips between arrays of plates including a transmitting plate on one side of the veneer and a juxtaposed receiving plate on the other. Phase plates on either side of and on the same level as the transmitting plate are empowered by a signal having the reverse phase to that applied to the transmitting plate. When wet veneer passes between the plate arrays, part of the transmitted signal is shunted reducing the signal received by the receiving plate. While this approach reduces sensitivity to vertical position of the veneer strips, eliminates problems associated with accidental grounding of the veneer and eliminates the need for mechanical contact with the veneer, in practice some unwanted signal shunting occassionally occurs as a veneer sheet initially enters the space between the transmitting and receiving plates, resulting in a false moisture detection signal. Also, the moisture detection apparatus disclosed in the prior application requires use of electronics equipment both above and below the veneer. The equipment mounted below the veneer tends to collect dust and debris which can effect moisture readings.
It would therefore be desirable to provide a moisture detection apparatus wherein moisture detection is not only substantially independent of thickness variations in the veneer and of the vertical position of the veneer between detector and transmitter plates but is also independent of the horizontal position of the veneer as it approaches the transmitting and detecting plates. Further, it would be desirable if the detection apparatus were mountable largely in a single package above the veneer so that dust and debris cannot collect on lower portions of the apparatus and thereby interfere with moisture readings.